Technical Abstract:
Studies of turbulent stratified flows in the surface layer near shelterbelts have been based on statistical descriptions for fields of velocity, temperature, and humidity under the assumption of constant air density. However, air density as a property of turbulence flow can be altered in time and space due to nonlinear interaction between fluctuations of the other meteorological parameters. Air density inhomogeneities in the surface layer may, along with the other mean and turbulence fields, contribute to mass and energy transfer. Measurements were taken in the vicinity of shelterbelts at the University of Nebraska-Lincoln Field Laboratory over a 5-day period in July 1994 to examine differences in small scale fluctuations of air density and its relation with turbulent fields and vertical profiles of mean temperature and mean wind speed. The shelterbelts consisted of two rows of alternating green ash, eastern red cedar, and Austrian pine of height approximately 12 m. A high-sensitivity optical refractometer was employed with a CSI data logger to measure gradient fluctuations of air refractive index from which fluctuations of air density were determined by use of the Gladston-Geil correlation and Taylor hypothesis. Temperature and velocity fluctuations were measured simultaneously using a 1-d sonic anemometer and a fine wire thermocouple. Vertical profiles of mean temperature and wind speed (10 levels) also were measured upwind and downwind of the shelterbelt. Time changes of high frequency fluctuations of air density were compared for 3 levels (0.6 m, 1.8 m, 3.0 m) at a windward distance of about 7H. Density fluctuations throughout the day at the level of 0.25 H and at 4 H in the lee were shown.